The present invention relates to the provision of reference signals to one or more radio heads.
Many wireless communication systems include one or more radio heads connected to a central unit by a wire based communications link, such as a T1 or E1 line. These high speed, high bandwidth lines typically do not include a separate clock line, but may nevertheless still be used to provide timing reference signals for phase locked loops within the radio heads. The phase locked loops are in turn used to perform a wide variety of tasks, such as carrier frequency synthesis. In general, the performance of the radio head, and, in particular, the accuracy of the carrier frequency transmitted by the respective transceivers, may vary depending on the timing accuracy of the timing reference signal provided to the radio head. Accordingly, it is desirable to supply accurate timing reference signals to the radio heads.
The present invention provides a method and apparatus for supplying a timing signal from a central unit to at least one radio head connected to the central unit via a communications link. An external signal, typically generated by a MSC, is received at a reference framer of the central unit and the reference framer generates a reference clock signal based thereon. This reference clock signal is used as the transmit clock for a second framer, known as the radio head framer. The radio head framer transmits a downlink signal to the radio head(s) via the communications link. The downlink signal may include payload information and an associated timing reference signal, typically in the form of an embedded clock signal. This timing reference signal of the downlink is based on the reference clock signal from the reference framer, rather than being based on a clock signal from an oscillator within the central unit. The transceivers within the radio head(s) may then control their RF transmissions using the timing reference signal of the downlink from the radio head framer of the central unit. Accordingly, the radio head(s) are provided with a timing reference signal that is based relatively directly on the signal from the MSC, thereby providing fewer opportunities for error introduction.